Paleoliquefaction in the Bajo Segura Basin (Eastern Betic Cordillera)

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ACTA GEOLOGICAHISPANICA, v. 36 (2001), nº 3-4, p. 233-244

Paleoliquefaction in the Bajo Segura basin (eastern Betic Cordillera)

Paleolicuefacción en la cuenca del Bajo Segura (Cordillera Bética oriental)

P.ALFARO(1 ) , J. DELGADO(1 ) , A. ESTÉVEZ(1 ) and C. LÓPEZ-CASADO(2 )

(1) Departamento de Ciencias de la Tierra y del Medio Ambiente. Universidad de Alicante. Apdo. 99. E-03080. Alicante (Spain). [email protected] / [email protected] / [email protected] (2) Departamento de Física Teórica y del Cosmos. Universidad de Granada. Av. Severo Ochoa s/n. Granada (Spain)

ABSTRACT

The Bajo Segura basin, in the eastern Betic Cordillera, displays a seismic activity characterized by small-magnitude earthquakes (1.5-4.5 mb), with some occasional moderate to high-magnitude events (> 5.0 mb). These earthquakes are produced by the activity of blind faults without surface ruptures. For this reason, the detection of paleoearthquakes in the geological record is limited to indirect evidence of paleoseismicity, mainly liquefaction features. Moreover, such evidence is abundant in the historical record of the 1829 Torrevieja earthquake, in some of its aftershocks and in the 1919 Jacarilla earthquake. In this study several layers of Holocene seismites previously described in the basin were analyzed, and correlated with various radiometric 1 4C datings. T h i s analysis enabled a recurrence period of approx. 1000 yr to be established for the moderate to high-magnitude earthquakes in the Bajo Segura basin.

Keywo rd s : Paleoliquefaction. Paleoseismology. Betic Cordillera. Bajo Segura basin.

RESUMEN

La cuenca del Bajo Segura, localizada en la Cordillera Bética oriental, tiene una actividad sísmica caracterizada principalmente por terremotos de pequeña magnitud (1.5-4.5 mb), aunque ocasionalmente han ocurrido terremotos de magnitud moderada-alta (> 5 mb). Estos terremotos están producidos por la actividad de fallas que no presentan ruptura en superficie (fallas ciegas). Por este motivo, el reconocimiento de paleoterremotos en el registro geológico se limita a las evidencias indirectas de paleosismicidad entre las que desta- can las estructuras de licuefacción sísmica. Además se tiene constancia de numerosas manifestaciones de licuefacción en el terremoto histórico de Torrevieja de 1829, en alguna de sus réplicas y en el terremoto de Jacarilla de 1919. En este trabajo se han analizado varios niveles de sismitas holocenas descritos previamente en la cuenca, y se han correlacionado con varias dataciones radiométricas de 14 C.

233 Este análisis ha permitido establecer un periodo de recurrencia de terremotos de magnitud moderada-alta en la cuenca del Bajo Segura, de aproximadamente 1000 años.

Pal ab r as cla v e : Paleolicuefacción. Paleosismicidad. Cordillera Bética. Cuenca del Bajo Segura.

IN T RO D U C T I O N available on the seismic liquefaction structures is s u fficiently detailed. Munson et al. (1995) gave an The Bajo Segura basin, situated in the eastern Betic approximate location for this based on the size of these Cordillera, displays seismic activity characterized by structures. Estimating the magnitude of paleo- small-magnitude earthquakes despite the occurrence of earthquakes, Ambraseys (1988) suggested that a occasional catastrophic events such as the 1829 minimum magnitude of 5 is required to trigger Torrevieja earthquake. Based on the damage caused by liquefaction, and above 5.5, their effects are relatively this earthquake, it has been estimated that the intensity at widespread. Recently, various methods have been used the epicenter reached the degree X (Muñoz et al., 1984). to estimate the magnitude of paleoearthquakes (see Other authors, on the basis of the spatial distribution of discussion in Obermeier, 1996). the damage, have estimated its magnitude as being between 6.3 and 6.9 Ms (Muñoz and Udías, 1991; The Bajo Segura basin contains sediments that are Delgado et al., 1993). Other historical earthquakes, highly susceptible to liquefaction. Seismites (s e n s u reaching an intensity of VIII, occurred at Guardamar del S e i l a c h e r, 1969) have been identified in the upper Segura (1523) and Jacarilla (1919). Given this low- Miocene (Montenat, 1980; Estévez et al., 1994) and moderate seismic activity, the information available for Quaternary deposits (Alfaro et al., 1996, 1999). evaluating the regional seismic hazard is scarce. For this M o r e o v e r, liquefaction occurred during the 1829 reason, there is considerable interest in any study of Torrevieja and 1919 Jacarilla earthquakes. This study paleoseismicity which expands the seismic catalog and analyses the susceptibility of the upper Pleistocene and allows the recurrence period of large earthquakes to be Holocene sediments filling the basin to liquefaction. In established with greater precision. addition, several layers of Holocene seismites, previously described by Alfaro et al. (1996), together In the Bajo Segura basin, the principal active faults with various radiometric 1 4C datings of the Holocene are blind with the result that they do not rupture the sediments (Soria et al., 1999), were used to establish the ground surface. In this region, the only information recurrence period for the moderate to high-magnitude available to date about paleoearthquakes is provided by earthquakes. seismic liquefaction features.

Obermeier (1996) made a detailed analysis of the TE C T ONIC SETTING current state of knowledge of the use of coseismic liquefaction features in studies of paleoseismology. Regional seismotectonic studies show that the Bajo These data can provide information about the position Segura basin (Fig. 1) is currently subject to a of the epicenter, the magnitude of the paleoseismic compressive stress field with a maximum horizontal event and the recurrence period of earthquakes of stress, trending NNW-SSE (Alfaro, 1995). This stress moderate-high magnitude. Paleoliquefaction studies field, which is linked to the convergence of the African have identified several paleoearthquakes that occurred and Eurasian plates, is consistent with the recent stress during the Holocene in different regions of the United tensor obtained by the SIGMA project corresponding to States, such as the New Madrid seismic zone (Saucier, the eastern part of the Betic Cordillera (Herraiz et al., 1991; Tuttle and Schweig, 1995) and the South 2000). Carolina coast (Obermeier et al., 1987; Amick et al., 1990; Amick and Gelinas, 1991). Based on dating of Compression has been active since at least the late some paleoearthquakes, recurrence times of moderate Miocene, and has deformed both the basement of the to high-magnitude earthquakes in the New Madrid Internal and External Zones of the Cordillera and the seismic zone have been established (Russ, 1979; Neogene-Quaternary sedimentary cover. A number of S a u c i e r, 1991). In addition, it is possible to determine folds formed in the Neogene-Quaternary sediments. the epicentral region provided that the information Some of these are associated at depth with various blind

234 Figure 1. Geological map of the study area showing location of boreholes and the main outcrops where seismites were observed. Figura 1. Mapa de localización geológica del área de estudio, en el que se han situado los principales afloramientos de sismitas y los sondeos estudiados. reverse faults and strike-slip faults. On the basis of the North, which is characterized by a more marked neotectonic and seismotectonic studies in the Bajo Segura subsidence, and by the La Mata-Torrevieja lagoons to the basin and in adjacent sectors (Montenat, 1977; Bousquet, South. 1979; López Casado et al., 1987; Somoza, 1993; Tab o a d a et al., 1993; Alfaro, 1995), the following faults stand out The Crevillente reverse fault, which runs N070E and as the most active: the Bajo Segura fault, the Crevillente dips northwards, is situated in the northern sector of the fault and the faults running NW-SE, particularly the San basin. The hanging wall is mainly composed of rocks Miguel de Salinas fault (Fig. 1). from the basement of the External Zones of the Betic Cordillera, whereas the footwall comprises sedimentary The ENE-WSW Bajo Segura fault is situated in the rocks from the late Miocene-Quaternary which show a southern part of the Bajo Segura basin. It is a blind progressive unconformity. Several anticlines which reverse fault which dips towards the South and is related, continued to fold during the Quaternary occur along this on the surface, to several asymmetrical anticlines in the fault (Alfaro, 1995). Neogene and Quaternary rocks. These folds include the Guardamar-El Moncayo, Benejúzar and Hurchillo folds In the Bajo Segura basin there are several NW- S E (Montenat, 1977; Taboada et al., 1993; Alfaro, 1995). On running faults, such as the Torrevieja, Guardamar and both sides of the anticlines lie two subsiding sectors San Miguel de Salinas faults. These right-lateral faults represented by the flood plain of the river Segura to the displaced several of the anticlines of the Bajo Segura.

235 SUSCEPTIBILITY OF HOLOCENE AND UPPER liquefaction, noting that it is limited to certain PL E I S T OCENE SEDIMENTS TO LIQUE F ACT I O N environments, whilst absent in others. Considering the data compiled by these authors, the susceptibility to Liquefaction occurred on repeated occasions in the liquefaction of the Segura valley sediments is high for Bajo Segura basin. The historical cases of liquefaction fluvial and littoral environments, and moderate for always took place in the fluvial and coastal sediments alluvial fan sediments. It should also be pointed out that, which lie close to the present-day course of the river. in alluvial fan sediments, the water table is at a greater This is a consequence of the susceptibility of these depth with the result that no cases of liquefaction have sediments to liquefaction. been documented for these sediments.

This susceptibility to liquefaction is due to a The age of the sediments is also a very important combination of a number of factors (Seed and Idriss, factor given that the susceptibility to liquefaction 1971; Youd and Perkins, 1978): (1) the presence of the decreases with age (Youd and Perkins, 1978). Kramer and water table close to the surface, which saturates the Arango (1998) quantified this and demonstrated that, all materials, (2) the fact that the sediments are fine-grained other factors being equal (geological, geotechnical etc.), and non-cohesive, and exhibit a reduced permeability the strength of sediments doubles at 0.1 Ma (Table 1). (fine or silty sand although recent data show that coarser Given that the surficial sediments (0-15 m) of the river sediments may also liquefy, Obermeier et al., 1993), and Segura valley are between 0 and 6000 years old (Alfaro, (3) the fact that these sediments are situated close to the 1995; Soria et al., 1999), this effect is negligible. surface (between 0 and -15 m), and, hence, are poorly Co n s e q u e n t l y , from a strictly geological point of view, the compacted. susceptibility of these sediments to liquefaction is high.

The water table in this valley is very shallow and The last factor which defines the susceptibility of linked to the river. Frequently, its maximum depth is less sediments to liquefaction is their mechanical properties. than 2 m although it increases towards the edges of the The granulometric curve of the sediment and its valley because of the rise in the topography (depths resistance to penetration (SPT) are particularly useful. greater than 10 m). Since the aquifer is characterized by The results of this test are related to the degree of its poor quality (high salinity), it is scarcely exploited sediment compaction and cementation, which is a direct with the result that it undergoes only small variations in indicator of the susceptibility of the sediment to level with time (Excma. Dip. Prov. Alicante and I.T.G.E., liquefaction. More than 70% of the SPT carried out in 1982). sediments from the Segura valley gave less than 10 blows, and SPT was similar to the rest of the valley. Fig. The most recent sediments filling the basin belong to 2 shows the results of the SPT tests carried out in sandy a unit of late Pleistocene-Holocene age. This unit sediments (fluvial and littoral) in the valley. The number consists of three different facies: (1) alluvial fan facies, of blows in this test increases owing to the existence of a (2) fluvial and lagoon facies and (3) littoral facies. greater confining pressure with depth. Likewise, the Given the nature of the sedimentary environments in the number of blows varies depending on the energ y valley (mainly alluvial, fluvial and littoral), fine sands ef ficiency of the equipment used (energy losses due to frequently occur. Youd and Perkins (1978) analyzed the friction of the test rods). In accordance with Seed and influence of the sedimentary environment on Idriss (1982), it is necessary to correct the result of the

Table 1. Liquefaction strength increase in relation to age increase (after Kramer and Arango, 1998). Quantities indicate the number of times soil strength increases with respect to strength of current sediments. Tabla 1. Aumento de la resistencia a la licuefacción con el aumento de la edad del sedimento (según Kramer y Arango, 1998). Las cantidades indican el número de veces que se incrementa la resistencia del terreno en comparación con sedimentos actuales.

Age (years) 0- 1 0 0 0 10 , 0 0 0 10 0 , 0 0 0 1, 0 0 0 , 0 0 0 St r e n g t h 1 1. 4 2 2 2. 4 5

236 SP T test for these two factors in order to obtain a correct evaluation of the strength of the sediment.

In Fig. 3 the granulometric curves of sandy sediments are given for several zones within the valley (the alluvial sediment tests were excluded). The curves were obtained using granulometric analyses of numerous samples taken from more than 100 boreholes. This figure shows that the particle-size distribution is not uniform: at the river mouth the fine content of the sand is very low (10%); close to the channel the fine content is 20-40% (although it can reach up to 60%); in the center and NW of the valley the fine content is 30-50%, and can reach 60-70%; Figure 2. Normalized SPT distribution in the sandy sediments la s t l y , in the NE of the valley the sand content is generally of the Vega Baja (Delgado, 1997). less than 30%, despite the existence of very localized pockets of very clean, fine sand (hence the wide range of Figura 2. Distribución del SPT normalizado en los sedimentos arenosos de la Vega Baja (Delgado, 1997). particle size in this area, see Fig. 3). Bearing in mind that the normalized SPT does not vary significantly, it can be deduced that the present-day river mouth and the areas close to the channel are most susceptible to liquefaction The largest manifestations of liquefaction occurred in the (high or very high), whereas the rest of the valley is immediate vicinity of San Fulgencio, being also abundant somewhat less susceptible (moderate to high) given that in Callosa del Segura (Larramendi, 1829). In the Callosa these sediments have a high silt and clay content, which del Segura area, the seismic liquefaction processes were confers cohesive properties upon the sediment and makes probably favored by the existing artesian conditions. In liquefaction more difficult. These results are consistent the vicinity of Callosa is a superficial sandy aquifer ar enas brujas with the historical data on liquefaction during the 1829 whose sands are colloquially known as Torrevieja earthquake (Larramendi, 1829). because of the frequent artesian processes. The remainder of the Bajo Segura basin comprises a multilayered aq u i f e r , and there is only evidence of small artesian LI QU E F ACTION DURING THE 1829 TOR R E V I E J A aquifers at more than 40 m depth. EA RT H Q UAK E Despite the extent of liquefaction processes in the The violence of the Torrevieja earthquake led to the past, there is currently no evidence on the surface liquefaction of recent sandy sediments, with frequent probably because of the small size of these liquefaction surface manifestations (Rodríguez de la Torre, 1984), features and the intense anthropogenic activity. In such as sand volcanoes, with craters of up to 12 cm in addition, historical documents describe the rapid removal d i a m e t e r, from which there was a prolonged and of the remains of sand volcanoes immediately after the abundant flow of water and sand. Similarly, there are earthquake (Rodríguez de la Torre, 1984). reports of numerous cases of cracks on the ground surface, with lateral spreading of blocks, from which, The earthquake was followed by a number of large again, there was an abundant flow of water and sand. aftershocks, some of which were large enough to trigger River banks underwent numerous breaks and collapses, liquefaction. Based on historical documents, therefore, probably related to the liquefaction of deep levels of sand we know that the aftershock of the 18/04/1829 event (Io = and lateral spreading of the non-liquefied part of the VII) led to the formation of nine sand volcanoes in the ground. The collapse of four bridges was due to this vicinity of Orihuela (Rodríguez de la Torre, 1984). phenomenon (Delgado et al., 1998). More recently, the 1919 Jacarilla earthquake (Io= According to the data compiled by Larramendi VIII; mb = 5.2; Mezcua and Martínez Solares, 1983) also (1829), the total area covered by sand extruded from sand caused liquefaction (Kindelán and Gorostízaga, 1920). volcanoes and fissures was over 7 km2 (Fig. 4). Most Although the information available does not allow the cases of liquefaction took place at the mouth of the geographical areas affected to be determined with Segura river, between Dolores, Benijófar and Guardamar. ce r t a i n t y , modeling of the soil behavior suggests that the

237 Figure 3. Fine grained content (f<0.072 mm) variation in the sandy sediments from the Segura valley.

Figura 3. Variación del contenido en finos (f<0.072 mm) en los sedimentos arenosos del Valle del río Segura.

238 Figure 4. Map showing the distribution of historical liquefaction during the Torrevieja earthquake (1829). Figura 4. Mapa en el que se muestra la distribución de la licuefacción histórica durante el terremoto de Torrevieja de 1829. phenomena only occurred close to Jacarilla (Delgado et attributed several soft-sediment deformation structures al., 1996). observed in the fluvial and littoral early Pleistocene sediments of Alicante and Guardamar del Segura to seismic shocks. All these soft-sediment deformation PA L E O L I QU E FACTION IN THE BAJO SEGURA structures, interpreted as seismites, indicate the BAS I N occurrence of earthquakes of moderate-high magnitude in the Bajo Segura basin, from the late Miocene to the early Some of the effects of paleoliquefaction processes Pleistocene. However, these seismites should not be used may be preserved in the geological record, displaying a to establish the recurrence of moderate-larg e series of sediment deformation structures called seismites paleoearthquakes in the region. In most cases, more (S e i l a c h e r , 1969). Estévez et al. (1994) describe soft- recent sediments are employed to establish these sediment deformation structures named detritic wedges recurrence intervals. and interpret them as seismites in the Tortonian deltaic sediments, outcropping between Crevillente and Elche. To this end, the last sediments (late Pleistocene- Montenat (1980) mentions the presence of seismites in Holocene) to fill the Bajo Segura basin were analyzed. Messinian shallow marine sediments, close to San Several trenches, which were excavated for the Miguel de Salinas. Finally, Alfaro et al. (1999) have construction of the Crevillente-Cartagena motorway,

239 were examined. Unfortunately, the reliance on trenches analyzed, no paleoseismic evidence was found. observation of trenches for paleoliquefaction studies In the study area, the only available evidence for constitutes a serious limitation in most of the basin. Th e recent paleoearthquakes is provided by several seismite water table is very shallow in the area where the layers identified in boreholes (Alfaro et al., 1996). sediments are highly susceptible to the production of Based on the analysis of core samples from various seismites. Only 1 to 2 m of sediments are exposed above boreholes drilled into the most recent sediments filling the water table; and the top meter or so has been reworked the Bajo Segura basin, these authors reported the by agricultural activity. Despite several hundred meters of presence of at least 25 liquefaction features interpreted

Figure 5. NW-SE simplified geological cross-section showing the morphology of the uppermost fill of the Bajo Segura basin (latest Pleistocene-Holocene), with location of boreholes, soft-sediment deformation structures interpreted as seismites (Alfaro et al., 1996), and the 14 C datings (after Soria et al., 1999).

Figura 5. Corte geológico NW-SE esquemático en el que se muestra la morfología del relleno más superficial de la cuenca del Bajo Segura (Pleistoceno terminal-Holoceno). Se han situado las columnas de los sondeos estudiados, las estructuras de deformación in- terpretadas como sismitas por Alfaro et al. (1996) y las dataciones radiométricas de 14 C (según Soria et al., 1999).

240 as seismites. These seismites belong to the last unit filling the Bajo Segura basin, dating from the late Pleistocene-Holocene (Soria et al., 1999). These authors

Figure 7. Age of soft-sediment deformation stru c t u r e s interpreted as seismites from 14 C datings. The solid line indicates the time interval in which seismites could have developed. The black solid rectangle indicates the most probable age deduced from isochrones. Figura 7. Edad de las estructuras sedimentarias de deforma c i ó n i n t e rpretadas como sismitas, deducida de las dataciones de 1 4C. La línea continua indica el intervalo de tiempo en el que se han podido originar cada una de las sismitas. El rectángulo n egro indica la edad más probable deducida a partir de las isocronas.

used 30 radiometric 14 C datings obtained in layers rich in organic material and shells (Quaternary Dating Research Center, Australian National University) to calculate the recent sedimentation rates in the basin and to establish the position of the isochrones over the last 8000 years at intervals of 1000 years. The young age of the latest sediments filling the basin, with a mean thickness of 30 m, is due to the high rate of sedimentation during the Holocene, varying between 3.7 mm/yr and 1.9 mm/yr, which is related to the last eustatic rise in the Mediterranean Sea (Soria et al., Figure 6. Soft-sediment deformation structures observed in the 1 9 9 9 ) . cores of boreholes. A. Microfaults. B. Convolute lamination. C. Fluid escape structure. We selected six of these boreholes (Fig. 5), where Figura 6. Esquemas de varias estructuras de deformación ob- the majority of these seismites were located and for servadas en los testigos de los sondeos. A. Microfallas. B. La- which 17 14 C radiocarbon datings were available in minación distorsionada. C. Estructura de escape de fluidos. an attempt to establish the recurrence intervals for

241 earthquakes of moderate-high magnitude in the study The only available evidence for recent area. These coseismic liquefaction features, which paleoearthquakes in the study area constitutes various liquefied when located at the surface, are found at a layers of liquefaction features that were identified from depth of nearly 40 m. The main types of seismites, core samples in sediments dating from the late limited above and below by non-deformed beds, are Pleistocene-Holocene. Analysis of these seismite layers microfaults, convolute lamination and fluid escape and the radiometric 1 4C datings corresponding to the structures (pillars) (Fig. 6). same boreholes enabled us to determine a recurrence interval of seismic events of moderate-high magnitude The possible age intervals for all of the seismites, of approximately 1000 years. Despite its limitations, the based on the radiometric 14 C datings, are shown in Fig. method allowed us to assess the recurrence of moderate- 7. The most probable age for each of the liquefaction high paleoearthquakes, using the information available features was established using the isochrones defined to date. by Soria et al. (1999). Based on the most probable age of each seismite, we identified at least seven paleoearthquakes over the last 8000 years. From these ACK N OW L E D G E M E N T S paleoearthquakes and the 1829 Torrevieja earthquake, we established a minimum recurrence interval of This research was financed by Research Project PB96-0327 earthquakes of moderate-high magnitude in the study of the Dirección General de Enseñanza Superior e area of approximately 1000 years. Although this Investigación. We thank Dr. Hibsch and Dr. Rodríguez Pascua methodology has certain limitations because of the for their helpful and interesting reviews. S. Steines reviewed the small size of the core samples (only 15 cm in diameter) English style of the paper. (Alfaro, 1996), it allows the identification of, at least, eight moderate to high-magnitude earthquakes in the Bajo Segura Basin during the last 8000 years. RE F E R E N C E S Seismites are also present below the 8000 yr isochrone and below one sample dated at 14,500 years, but the Al f aro, P., 1995. Neotectónica en la Cuenca del Bajo Segu r a scarcity of datings prevents us from making a reliable (Cordillera Bética oriental). Tesis doctoral, Universidad de estimate for the recurrence interval in these older Alicante, 219 p. d e p o s i t s . A l faro, P., Domènech, C., Estévez, A., Soria, J.M., 1996. E s t ructuras de deformación en sedimentos del Cuatern a r i o reciente de la cuenca del Bajo Segura (Alicante). CO N C L U S I O N S Discusión sobre su posible origen sísmico. Geogaceta, 17, 9 1 - 9 4 . In the Bajo Segura basin, situated in the eastern part Al f aro, P., Estévez, A., Moretti, M., Soria, J.M., 1999. Struc t u r e s of the Betic Cordillera, no direct evidence of sédimentaires de déformation interpretées comme séismites paleoearthquakes is available since the principal active dans le Quaternaire du bassin du Bas Segura (Cordillère faults are blind thrusts and blind strike-slip faults. bétique orientale). C. R. Acad. Sci. Paris, 328, 17-22. Evidence of paleoseismicity is restricted to coseismic Am b r a s e ys, N., 1988. Engineering seismology . Earth q u a k e Eng. liquefaction features with the result that indirect St r uctural Dynamics, 17, 1-105. evidence of paleoearthquakes is of particular interest in Amick, D., Gelinas, R., 1991. The search for evidence of large this region where surface fault ruptures are not visible. prehistoric eart h q u a kes along the Atlantic seaboard. Such references to paleoliquefaction have been Science, 251(4994), 655-658. reported during various historical earthquakes, Amick, D., Gelinas, R., Maurath, G., Cannon, R., Moore, D., e s p e c i a l l y, in the case of the 1829 To r r e v i e j a Billington, E., Kemppinen, H., 1990. Paleoliquefaction earthquake. features along the Atlantic seaboard. US Nuclear R eg u l a t o ry Commission Repport NUREG CR-5613, In the Bajo Segura basin, analysis of trenches in the 146 p. most recent Holocene sediments filling the basin is Bousquet, J.C., 1979. Quatern a ry strike-slip faults in limited by the shallowness of the water table between 1 So u t h e a s t e r n Spain. Tec t o n o p h ysics, 52, 277-286. and 2 m in depth. It should also be pointed out that De l g ado, J., Giner, J.J ., Jaúregui, P., López-Casado, C., Estévez , these sediments have been reworked by agricultural A., 1996. Generación automática de mapas de daños a c t i v i t y. sismoinducidos. Geogaceta, 20(5), 1170-1172.

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243 S egura Basin (eastern Betic Cordillera, Spain): eustatic Tuttle, M.P., Schweig, E.C., 1995. A r c h e o l ogical and implications. Bull. Soc. géol. France, 170(3), 349-354. pe d o l o gical evidence for large prehistoric earth q u a k es in the Taboada, A., Bousquet, J.C., Philip, H., 1993. Coseismic elastic Ne w Madrid seismic zone, central United States. Geology , models of folds above blind thrusts in the Betic Cordilleras 23(3), 253-256. (Spain) and evaluation of seismic hazard. Tec t o n o p hy s i c s , You d , T.L., Perkins, D.M., 1978. Mapping of liquefaction induced 220, 223-241. ground failure potential. J. Geotech. Eng., 113, 1374-1392.

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